Phase shift keying (PSK) is a form of modulation where the phase of a carrier is changed by binary data, and demodulation is accomplished by comparing the received signal to a fixed reference. In binary phase shift keying (BPSK), the modulator output has two phases. It is either in-phase or 180.degree. out-of-phase, with the local oscillator, depending on whether the input is a logic one or zero. To demodulate a BPSK signal it is necessary for the demodulator to establish a reference phase angle. This reference is compared to the phase angle of the received signal to determine whether the received signal represents a one or a zero.
An alternative to PSK is differential phase shift keying (DPSK). In a DPSK signal the information is contained in the differential phase between consecutive bit intervals. With this technique, the received signal is demodulated by comparing the carrier signal phase in each bit interval with the phase during the immediately preceding bit interval, the latter serving as a reference. Binary DPSK, which is the most popular form of DPSK, uses two phase angles to represent the data. Although DPSK is easier to implement than PSK, DPSK requires a greater signal-to-noise ratio to achieve the same error rate as PSK.
Quadrature DPSK modulation (the use of eight phase angles to represent data) is discussed in U.S. Pat. No. 3,368,036, entitled "Demultiplexing and Detecting System for Predicted Wave Phase-Pulsed Data Transmission System" issued to R. C. Carter et al. The patented invention permits two bits of binary data to be transmitted and received per bit interval because with quadrature DPSK the carrier signal in a given bit interval can assume any one of four different phases with respect to the immediately preceding bit interval.
FIG. 1 illustrates a prior art DPSK demodulator that uses a delay element (.DELTA.t) at the intermediate frequency. The delay element must be equal to one bit period and must represent a whole number of carrier cycles. The output of the delay element is multiplied or mixed with the undelayed intermediate frequency signal to synchronously demodulate the signal. In mathemathical terms the intermediate frequency signal will be +sin .omega..sub.c t or -sin .omega..sub.c t. Since the mixer in FIG. 1 produces the product of two successive values of the intermediate frequency input signal, the output signal thereof will be sin.sup.2 .omega..sub.c t=(1/2)(1-cos 2.omega..sub.c t) when two consecutive phases are the same, or -sin.sup.2 .omega..sub.c t=-(1/2)(1-cos 2.omega..sub.c t) when two consecutive phases are different. The low-pass filter elminates the double-frequency component, yielding either a positive or negative dc value. The dc value is then one-bit-quantized by a zero crossing detector or comparator. This detection scheme offers simplicity in concept but is extremely difficult to implement because the delay element must have a very accurate delay, no more than .+-.10 degrees of phase shift at the intermediate frequency.
FIG. 2 illustrates another prior art DPSK detector wherein the intermediate frequency signal is reduced to quadrature baseband components, labeled I and Q; the baseband components are then converted to a phase angle via an arc-tangent function. The result from the arc-tangent function is delayed and compared to determine the phase difference, which represents the desired data. The analog/digital interfaces are shown in FIG. 2 as occurring after baseband conversion and filtering so that the arc-tangent function can be done in a primarily digital circuit.
Another prior art DPSK detector is illustrated in FIG. 3. In the FIG. 3 detector the arc-tangent function, which is quite difficult to implement, is eliminated in favor of a delay, multiply, and add circuit that provides simpler circuitry with substantially similar performance to the FIG. 2 embodiment. Both of the FIG. 2 and FIG. 3 embodiments can be characterized as amplitude-quantized in that they are sensitive to the amplitudes of the quadrature baseband components. Thus for digital processing a pair of analog-to-digital converters are necessary to implement the circuits.